As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Based on kinds of external devices in which the secondary battery is used, the secondary battery may be used in the form of a single unit cell or in the form of a battery pack including a plurality of unit cells electrically connected with each other. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the output and the capacity of a single unit cell. On the other hand, medium- or large-sized devices, such as lap-top computers, electric vehicles, or hybrid electric vehicles, require use of a battery pack because high output and large capacity are necessary for the medium- or large-sized devices.
The battery pack is a battery structure in which a plurality of unit cells are arranged in series and/or in parallel with each other and electrically connected with each other in regular sequence. It is very important to maintain the arrangement structure of the unit cells in the battery pack.
When the battery pack is generally constructed, cylindrical secondary batteries, prismatic secondary batteries, or pouch-shaped secondary batteries may be used as the unit cells. When the prismatic secondary batteries or the pouch-shaped secondary batteries are used as the unit cells, the batteries can be easily stacked one on another, and the arrangement structure of the batteries is stable, whereby there is no problem in electrically connecting the batteries with each other. However, the cylindrical secondary batteries, which have greater capacity than the prismatic secondary batteries or the pouch-shaped secondary batteries, have great difficulty in maintaining the arrangement structure due to the appearance characteristics thereof.
For this reason, there has normally been used a method of securely connecting cylindrical unit cells to a nickel plate by soldering or spot welding and wrapping the connected cylindrical unit cells with tape or coating the connected cylindrical unit cells with plastic so as to maintain the arrangement structure of the cylindrical unit cells, thereby constructing a battery pack.
However, the strength of the tape and the plastic coating is small, and therefore, the possibility is very high that the tape and the plastic coating are damaged or broken. Consequently, the tape and the plastic coating are not suitable for stably maintaining the arrangement structure of the unit cells. In addition, when the battery pack is constructed using the tape, a process for detaching and attaching the tape is complicated and troublesome, which is a factor that increases the time necessary for constructing the battery pack.
Also, the battery pack is generally constructed in the form of a hard pack manufactured by mounting a protection circuit to a core pack having unit cells arranged therein and wrapping the outer surface of the core pack with an outer case. When the hard pack is constructed with the cylindrical unit cells, a device for maintaining the arrangement structure of the cylindrical unit cells as well as the outer case for stably mounting the protection circuit are used, which increases the manufacturing costs and the process time.
Consequently, various types of fixing devices for maintaining the arrangement structure of cylindrical batteries have been developed. For example, Korean Patent Application Publication No. 1995-10160 discloses a fixing device including a spacer (a middle spacer) positioned in the middle of cylindrical batteries and a coupling member for integrally wrapping the outer surfaces of the batteries while the batteries are located at the middle spacer. In the disclosed fixing device, however, the middle spacer and the coupling member are separately manufactured, and therefore, the assembly process is troublesome. Also, when the coupling member is not mounted in position, the coupling member may be separated from the middle spacer.
Japanese Patent Application Publication No. 2000-100401 discloses a middle spacer similar to the aforesaid spacer. However, this middle spacer only serves to space the cylindrical batteries from each other. In order to fix the cylindrical batteries, it is necessary to fix electrode terminals of the cylindrical batteries to a circuit board using an electrode connection plate, as shown in FIG. 1 of the publication. Consequently, the use of this spacer is restricted, and the assembly process is very troublesome.
In addition, U.S. Patent Application Publication No. 2001-46624 discloses a fixing device including a pair of upper and lower covers in which pluralities of cylindrical batteries are mounted in a two-layer structure, a middle cover, and a plurality of holding ribs formed at the insides of the covers. The fixing device has an advantage in that the cylindrical batteries are stably mounted; however, the fixing device has a disadvantage in that the cylindrical batteries are supported on the covers by the holding ribs, and therefore, when the strength of the ribs is not sufficient, the structural stability of the fixing device is lowered. Especially, the middle cover does not provide sufficient mechanical strength due to the structural characteristics thereof.
Consequently, there is very high necessity of a structural device that is capable of stably mounting a protection circuit without an additional outer case while maintaining the arrangement structure of the cylindrical unit cells.